A variety of treatments for leg length discrepancies are known. Leg length discrepancies may arise from birth defects, improper bone growth, disease, or trauma. Such treatments of leg length discrepancies include the use of shoe lifts and special boots to raise the foot in the equinus position. These treatments are simple, but often uncomfortable and aesthetically unattractive, especially in larger leg length discrepancies of 4 cm or more. The field of orthopedics includes other techniques, such as stimulating epiphyseal growth, surgical shortening of the longer limb, and surgical lengthening of the short limb.. As these techniques have developed, the trend has shifted to limb lengthening techniques to treat leg length discrepancies.
Limb lengthening techniques require that the bone of the limb be cut, called an osteotomy or corticotomy. The bone begins development of a callus at this location. The two bone portions are then pulled apart by a mechanical device that is surgically attached to the bone. This procedure is called a distraction, in which the callus is stretched, thereby lengthening the bone.
The current mechanical devices used for limb lengthening are external fixators transcutaneously connected to the bone using wires, pins, or screws. These methods cause such complications as infections at the points of the transcutaneous connections, discomfort in wearing the fixator for the patient, and the unattractive appearance of the fixator. These complications become most evident when the devices are used to lengthen a femur. These problems may be avoided by implanting an internal fixation device inside the bone to perform the distraction.
The "state-of-the-art" limb lengthening technique applies Ilizarov's principle of tension-stress using the Ilizarov external ring fixator with transfixion wires. (Ilizarov, G. A. Clinical application of the tension-stress effect for limb lengthening, Clin Orthop Rel Res, 1990; 250:8-26). According to the principle of tension-stress, living tissue subjected to slow, steady tension becomes metabolically activated. Hence, upon the creation of a bone gap and a subsequent distraction of the gap, new bone may be formed to generate an increase in length.
Ilizarov's research indicates that four objectives are especially important in facilitating optimal bone healing. These are:
1. Preservation of the blood supply of the fracture site and the limb as a whole; PA1 2. Preservation of the osteogenic tissue (periosteum, endosteum, and bone marrow) during osteosynthesis and postoperative care; PA1 3. Functional activity of the muscles and joints of the limb; and PA1 4. Early patient mobilization. PA1 1. corticotomy to preserve marrow and periosteum; PA1 2. stable external fixation; PA1 3. delay before distraction of 5-7 days; PA1 4. distraction rate of 1.0 mm per day; PA1 5. distraction frequency of at least 4 times per day; PA1 6. a period of neutral fixation after distraction; PA1 7. normal function of limb.
Other important objectives are to completely reduce the bone fragments and to provide rigid fixation. Combining these objectives with the principle of tension-stress allows formation of bone not only to heal fractures, but also to correct deformities. The treatment of fractures, pseudarthroses, joint contractures, achondroplasia, and limb length discrepancy are achieved through treatments incorporating these objectives.
Ilizarov's external ring fixation system employs tensioned transfixion wires that pass through the skin and attach to the bone. Based on a study of canine tibiae using this device, Ilizarov formulated the following procedures for maximizing the effectiveness of treatment:
The application of these principles may vary according to individual differences and specific characteristics of bone formation. The rate and frequency of distraction may need to be changed during treatment according to the quality of bone being formed. However, these principles are the framework under which most procedures are performed.
The Ilizarov method has been quite successful, but with numerous complications. The device causes serious discomfort for the patient, especially when the device is applied to the femur. The procedure also involves numerous wires that pass through the skin, creating possible infection sites.
The prior art also includes the method of DeBastiani et al., termed callotasis, which is similar to the biological method of Ilizarov. (DeBastiani, G., Aldegheri, R., Renzi-Brivio, L., Trivella, G. Limb lengthening by callus distraction (callotasis), J Ped Orthop 1987;7(2):129-134). Instead of a ring fixator, DeBastiani et al. uses a telescoping dynamic axial fixation system where an external monolateral frame is attached to the bone by means of screws. Distraction is performed between 10-15 days after a diaphyseal corticotomy at the rate of 0.25 mm every 6 hours. Upon completion of the desired lengthening amount, the locking screws are removed when consolidation of the callus is determined radiographically. The fixator is removed when corticalization of the new bone is achieved. With this device, DeBastiani et al. reported on 100 lengthenings with a total complication rate of 14%.
Wagner developed a technique which focused on a method of filling the distraction gap with bone. (Wagner, H. Operative lengthening of the femur, Clin Orthop Rel Res 1978;(136):125-142). Four Schanz screws are inserted into a bone and attached to a monolateral external fixator. A diaphyseal transverse osteotomy is performed between the 2 sets of screws. The patient carries out distraction at the rate of 1.5 mm per day by turning screws on a monolateral square telescoping apparatus. When the lengthening amount is achieved, the bone is then stabilized with internal fixation and grafted with iliac bone. The distraction apparatus and the Schanz screws are then removed.
Monticelli and Spinelli describe another method called distraction epiphysiolysis that is used to lengthen limbs in patients who are still in the growth stage. (Monticelli, G., Spinelli, R. Distraction epiphysiolysis as a method of limb lengthening. Experimental Study, Clin Orthop Rel Res 1981;(154):254-261). This method involves creating a fracture across the epiphyseal plate using a high level of traction. Gradual distraction is accomplished through two pins placed in the epiphysis and the diaphysis. This procedure has the advantages of minimal surgical difficulties and a relatively short treatment time. However this procedure involves a traumatic sudden pain and frequent premature closure of the growth plate.
The above procedures all involve the use of external fixation. Use of external fixation devices have been successful, but have many inherent problems, the most obvious disadvantage being discomfort for the patient. Internal distraction devices avoid many of these problems.
German literature contains the first reports of internal distractors. Schollner reported on a modification to the technique of Anderson by using a distraction device implanted adjacent to the bone being lengthened. (Schollner, D. New ways of operating to lengthen the femur, Z. Orthop. 1972;110:971-974 citing Anderson, W. V. Leg lengthening, J Bone Joint Surg [Br] 1952;34-b:150). Gotz and Schellmann reported experimental studies on a hydraulic distractor placed in a modified interlocking -nail. (Gotz, J. Schellmann, W. D. Continuous lengthening of the femur with intramedullary stabilization, Arch Orthop Unfall-Chir 1975;82:305-310). Their device employed a cylinder external to the bone that supplied hydraulic pressure to an internal nail. Baumann and Harms developed a telescoping nail driven by a threaded spindle transcutaneously attached to the nail. (Baumann, F., Harms, J. The extension nail. A new method for lengthening of the femur and tibia, Arch Orthop UnfallChir 1977;90:139-146). Each of these devices employs a connection from an internal device to an external means to drive the distractor.
Witt, et al. is the first report in the prior art on human clinical results from a totally implantable femur distractor. (Witt, A. N., Jager, M., Bruns, H., Kusswetter, W., Hildebrant, J. J. Die operative Oberschenkelverlangerung mit einem vollimplantierbaren Distraktionsgerat, Arch Orthop Traumat Surg 1978;(92):291-296). Witt, et al. discloses a device implanted in the soft tissue adjacent to the bone and screwed into the femur proximally and distally. The device of Witt, et al. employs an electric motor housed in the device to generate a distraction force. The motor is controlled by telemetry from outside the body, providing for both forward and backward motion. Witt does not disclose a device implantable in the bone itself or the use of a shape memory alloy to achieve lengthening.
The prior art also includes Bliskunov. (Bliskunov, A. I. An implantable apparatus for lengthening the femur without external drive, Med Tekhnika 1984;(2):44-49.) Bliskunov discloses a distractor for intramedullary implantation. Bliskunov discloses a long rod with a rotary ratcheting mechanism that is inserted into the medullary canal following a partial osteotomy. The device of Bliskunov requires a lever is hinged to the long rod and screwed into the wing of the ilium. Bliskunov requires a hip rotation of at least 15.degree. to turn the ratchet wheels of the drive mechanism to achieve lengthening. Each rotation produces 0.04 mm of lengthening, with the movements repeated daily according to a prescribed lengthening rate. Lengthenings of up to 12 cm have been achieved using the Bliskunov device.
Also included in the prior art is international patent WO 91/00065 to Betz et al., filed Jul. 4, 1989, issued Jan. 10, 1991. Betz et al. discloses a fully implantable intramedullary system for lengthening, using telemetry to control an electric motor. Betz et al. postulated that placement of a device in the marrow cavity would not influence healing of a fracture or formation of bone due to callus distraction. Betz et al. reasoned that the nail would not affect the periosteum and that the blood supply comes from the medullar vessels that form around the nail. Betz et al. developed two variants of an intramedullary nail, one with implanted energy and control units, and one with external energy and control units. In the first device, a battery pack and a telemetry receiver are implanted subcutaneously, with an automatic controller. In the second device, only a receiver is implanted and connected to the driving motor, allowing for a much smaller subcutaneous packet. The patient attaches a telemetry sender to their leg during the night, which activates the device and transmits the energy to the motor. Both devices require an electric motor to provide a distraction force. Betz et al. does not disclose a shape memory alloy for providing a distraction force or a ratcheting system for regulating lengthening events.
Guichet et al. developed a device similar to that of Bliskunov, but did not use a lever arm to produce a distraction. (Guichet, J. M., Grammont, P. M., Trouilloud, P. Intramedullary Nail for Gradual Limb Lengthening: Animal Experimentation, Trans Orthop Res Soc 1991;16:657). The device of Guichet et al. consists of a stainless steel intramedullary nail with two telescoping tubes connected by a rotary ratcheting mechanism. Guichet requires knee rotations of 30.degree. to trigger the rotary ratchet mechanism, resulting in 0.1 mm of lengthening, with a daily rate of distraction set at 1.24 mm. His initial report described in vivo trials in 10 sheep, completing an average of 6.3 cm of lengthening over 32 days.
U.S. Pat. No. 5,156,605 to Pursley, filed Jun. 11, 1991, issued Oct. 20, 1992, describes an Automatic Internal Compression-Distraction-Method and Apparatus. Pursley discloses two embodiments of an intramedullary telescoping distractor. Like the device of Betz et al., both embodiments require the use of an electric motor and controller to provide a distraction force. Pursley describes two embodiments where the motor and controller are used to drive a lead screw. In the first form, the motor is housed outside the body, and connected to the internal tube by means of a flexible shaft. In the second form, the motor and control units are internally mounted, and controlled by a communication assembly from outside the body. No reports of clinical or experimental use of this device have been found in the literature.
Other limited reports of work on internal lengthening devices include Herzenberg, J. E., Hensinger, R. N., Goldstein, S. A. Michigan intramedullary leg lengthening nail, In: Biomechanics, Trauma and Sports Medicine Laboratory Annual Report, University of Michigan, 1989, Verkerke, G. J., Koops, H. S., Verb, R. P. H., Nielsen, H. K. L. Design of a load cell for the Wagner distractor, Proc Instn Mech Engrs 1989;203:91-96, Fisher, C. Personal communication. Feb. 12, 1992, and Hellend, P. Femoral elongation by use of an elongable intramedullary device, Acta Orthop Scand 1992;63(Suppl 247):16. The following table is a summary of research work done on internal lengthening devices with implantable drive mechanisms.
______________________________________ NAME YEAR DRIVE MECHANISM ______________________________________ Witt, et al. 1978 Electric Motor Bliskunov 1984 Hip Rotation Herzenberg, et al. 1989 N/A Verkerke, et al. 1989 Electric Motor (endoprosthesis) Betz, et al. 1990 Electric Motor Fisher, et al. 1992 Electric Motor Guichet, et al. 1991 Hip Rotation Hellend 1992 Leg Rotation Pursley, et al. 1992 Digital Motor ______________________________________
While the prior art shows various intramedullary nails for limb lengthening, the prior art does not include the use of shape memory alloys as a expansion force generation means. To trigger lengthening events, the prior art has relied on mechanical input from the patient or an electric motor to provide a distraction force. In the case of mechanical input, this reliance may require a transcutaneous connection or additional connections to bones other than the bone being lengthened to provide mechanical displacement. The use of a motor to provide distraction force may require that a larger package be implanted. The use of a shape memory alloy to provide a distraction force may avoid these disadvantages.
A shape memory alloy (SMA) is a material having an ability to "remember" a certain shape after significant deformation. Two phases of materials possessing the shape memory effect, austenite and martensite, provide for this property. These phases have significantly different moduli of elasticity and occur at different temperatures. Martensite is a low temperature, low modulus phase. Austenite is a higher temperature, high modulus phase.
Fabrication of an alloy possessing the shape memory effect is a combination of mechanical deformation and temperature change. The "memory shape" of the alloy is set in the austenite phase. The alloy is subsequently cooled to the martensite phase and deformed. Subsequent heating of the alloy restores the original memory shape and recovers the deformation. The alloy is under high strain in the martensite phase, and under low strain in the austenite phase.
A two-way shape memory effect (TWSM) occurs when a SMA returns to the deformed martensite shape upon cooling from austenite. A special treatment process called training biases the martensite structure to form the same shape each time upon cooling, resulting in the TWSM effect. As a result, the SMA may not require a resetting force to achieve cycling as does the "one-way" alloy.
Whereas the one-way and two-way effects are thermally activated, superelasticity is a mechanical type of shape memory. One method of fabricating a superelastic material is to deform the material in an austenire phase, above the martensite start temperature (M.sub.s). The resulting stress causes the martensite phase to become more stable than the austenite phase and results in a martensite transformation, called stress-induced martensite (SIM). When the stress is released, the martensite becomes unstable and the original shape is fully recovered.
A number of industries use shape memory alloys. One popular application of shape memory alloys is in tube couplings. Medical applications such as guide wires and orthodontic arch wires use shape memory alloys because of their properties of superelasticity and high shape recovery, and because of their ability to provide low, continuous forces. The ability of shape memory alloys to exert a force after implantation and exposure to body temperature make them useful in orthopedic applications such as staples, screws, nails, and femoral cups.
Shape memory alloys provide distinct advantages over the prior art. Biocompatible shape memory alloys are available. Furthermore, shape memory alloys that provide sufficient force for distraction may be contained within a small package within the intramedullary nail apparatus. Shape memory alloys may be molded to fit within necessary parameters.
It is therefore a motive of the invention to provide for an intramedullary nail apparatus that may be encapsulated, provides a shape memory alloy for an expansion force and a drive mechanism for transmitting motion and force to accomplish distraction of a bone, and that may be electrically activated without the need for mechanical input from a patient.